Operational amplifiers are almost always used with negative feedback, in which part of the output signal is returned to the input in opposition to the source signal. (It is also possible to have positive feedback, in which the signal returned to the input aids the original source signal. However, negative feedback turns out to be more useful in amplifier circuits.) Frequently, we analyze op-amp circuits by assuming an ideal op amp and employing a concept that we call the summing-point constraint.